Iron sulfide mixtures; iron sulfide heavy metal treating agents; and methods of treating using such agents

ABSTRACT

Novel iron sulfides having excellent durability and excellent treating properties of heavy metals, processes for producing the iron sulfides, iron sulfide mixture, a heavy metal treating agent containing either of these novel iron sulfides as an effective component, and a method by which wastes containing various heavy metals are treated with the heavy metal treating agent are disclosed. The iron sulfide having a mackinawite structure which contains FeM x N y S z  wherein M represents an alkaline earth metal, N represents an alkali metal, and x, y and z, indicating the molar proportions of the respective elements, represent numbers satisfying 0.01&lt;x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component.

This is a divisional of application No. 10/053,674 filed Jan. 24, 2002,now U.S. Pat. No. 6,682,713; the disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to novel iron sulfides having excellentdurability, processes for producing the same, iron sulfide mixture, aheavy metal treating agent containing either of these novel ironsulfides as an effective component, and a method by which wastescontaining various heavy metals are made harmless with the heavy metaltreating agent.

DESCRIPTION OF THE RELATED ART

Reviews of Pure and Applied Chemistry, Vol.20, pp.175-206 (1970) showsstructures of iron sulfides. It discloses that the generally known ironsulfides for industrial use and the iron sulfide produced by melting amixture of an iron powder and sulfur have a pyrrhotite structure, whilethe iron sulfide yielded by mixing a solution containing iron(II) ionswith a solution containing sulfur ions has a mackinawite structure.

A technique in which wastes containing various heavy metals are madeharmless with an iron sulfide is widely known. For example, a method fortreating harmful heavy metals, e.g., Pb, Cd, Cr, Hg and As, in anaqueous solution with an iron sulfide for industrial use (pyrrhotitestructure) is disclosed in, e.g., Japanese Patent Publication No.43472/1974 and Japanese Patent Laid-Open Nos. 31806/1972, 13294/1975,96053/1975, 126685/1977 and 227881/1985.

A technique is also known in which an iron sulfide (mackinawitestructure) prepared by mixing a solution containing iron(II) ions with asolution containing sulfur ions is used to treat heavy metals. The ironsulfide obtained by this process is known to have the higher ability totreat heavy metals than iron sulfides for industrial use. For example,Japanese Patent Laid-Open Nos. 11291/1973, 31152/1974, 113559/1977,148473/1977 and 102273/1978 disclose a method of treating harmful heavymetals with a solution containing iron(II) ions and a solutioncontaining sulfur ions or with an iron sulfide obtained by mixing thesesolutions.

However, the iron sulfide having a mackinawite structure is sosusceptible to oxidation that it reacts with moisture and oxygen in theair and thereby decomposes into sulfur and iron(III) hydroxide. Namely,the ability of this iron sulfide to treat heavy metals readilydecreases. Because of this, treatment with the iron sulfide having amackinawite structure has hitherto been conducted generally in such amanner that a solution containing iron (II) ions is mixed with asolution containing sulfur ions to prepare a slurry containing the ironsulfide and this slurry is immediately mixed with a waste to be treated,such as a wastewater. Although the solution containing sulfur ions whichis usually employed is an aqueous solution of sodium sulfide or sodiumhydrosulfide from the standpoints of cost and industrial availability,it is necessary to handle the solution by a skilled person having aknowledge of chemistry because of the harmfulness, corrosiveness, andoffensive odor of the solution, etc. Namely, the solution is generallydifficult to handle. Furthermore, there has been the following problem.In the case where the iron sulfide is used as a slurry, heightening theconcentration of the iron sulfide is difficult because of the necessityof imparting a certain level of flowability. This limitation on ironsulfide concentration results, for example, in an increasedtransportation cost when the iron sulfide slurry is produced in, e.g., afactory. Moreover, when the iron sulfide is added in a large amount totreat a fly ash or soil containing heavy metals in a high concentration,the waste thus treated has too high a water content and is hencedifficult to handle thereafter, although this problem is not aroused inthe treatment of wastewaters and the like.

On the other hand, in the case where a powder of mackinawite ironsulfide is prepared from the slurry through filtration and drying, therehave been the following problems. It is necessary to conduct theoperation in an inert atmosphere or to add an antioxidant in order toprevent oxidation. The iron sulfide powder obtained should be stored ina container impermeable to oxygen and moisture in order to prevent thepowder from oxidatively deteriorating. Alternatively, it is necessary toadd a reducing agent to the powder so as to prevent oxidation. Eventhough a reducing agent is added, this does not basically eliminate thesusceptibility to oxidative deterioration and the powder still hasconsiderably poor storage stability because the iron sulfide begins tooxidize when the reducing agent has been consumed.

SUMMARY OF THE INVENTION

The invention has been made under the circumstances described above.

One object of the invention is to provide novel iron sulfides which haveexcellent durability and are highly effective in treating heavy metals.

Another object of the invention is to provide processes for synthesizingthese novel iron sulfides.

Still another object of the present invention is to provide iron sulfidemixture.

Further object of the invention are to provide a heavy-metal treatingagent comprising either of the iron sulfides as an effective component.

Still another object of the invention is to provide a method by whichheavy metals contained in an ash, soil, wastewater or the like are madeharmless with the treating agent.

Intensive investigations have been made to overcome the above-describedproblems, i.e., the drawback of the synthetic iron sulfides heretoforein use that those have poor durability although highly active in heavymetal treatment. As a result, it has found that the durability of aniron sulfide can be greatly improved by incorporating an alkaline earthmetal into the iron sulfide at least in a given amount to convert theiron sulfide into an iron sulfide which contains an essential componenthaving a novel composition represented by FeM_(x)N_(y)S_(z) (wherein Mrepresents an alkaline earth metal, N represents an alkali metal, and x,y, and z, indicating the molar proportions of the respective elements,represent numbers satisfying 0.01<x≦0.5, y≦0.2, and 0.7≦z≦1.4). It hasbeen further found that the novel iron sulfide is obtained by mixing anaqueous solution of a salt of bivalent iron, an aqueous solutioncontaining sulfur ions and an alkaline earth metal ingredient andadjusting the pH of the resultant slurry to 7.0 or higher. Furthermore,it has been found that a heavy metal treating agent comprising thisnovel iron sulfide as an effective component is far more effective intreating various heavy metals than the synthetic iron sulfidesheretofore in use and pyrrhotite iron sulfide. The invention has beencompleted based on these findings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the results of analyses by X-ray diffractometry of the ironsulfide powder prepared in Example 1 and of this powder which hadundergone a one day durability test at 70° C. and 70% RH, in which Xaxis (abscissa) indicates angle of diffraction 2θ (degree) and Y axis(ordinate) indicates the intensity of X-ray (unit: cps);

FIG. 2 shows the results of analyses by X-ray diffractometry of the ironsulfide powders prepared in Example 1, Example 2 and Example 3. The Xaxis (abscissa) indicates angle of diffraction 2θ (degree) and the Yaxis (ordinate) indicates the intensity of X-ray (unit: cps);

FIG. 3 shows the results of analyses by X-ray diffractometry of the ironsulfide powder prepared in Comparative Example 1 and of this powderwhich had undergone a one day durability test at 70° C. and 70% RH, inwhich X axis (abscissa) indicates angle of diffraction 2θ (degree) and Yaxis (ordinate) indicates the intensity of X-ray (unit: cps);

FIG. 4 shows the results of analyses by X-ray diffractometry of the ironsulfide powder prepared in Comparative Example 3 and of this powderwhich had undergone a one day durability test at 70° C. and 70% RH, inwhich X axis (abscissa) indicates angle of diffraction 2θ (degree) and Yaxis (ordinate) indicates the intensity of X-ray (unit: cps);

FIG. 5 shows the results of analyses by X-ray diffractometry of the ironsulfide powder prepared in Example 5 and of this powder which hadundergone a one day durability test at 70° C. and 70% RH, in which Xaxis (abscissa) indicates angle of diffraction 2θ (degree) and Y axis(ordinate) indicates the intensity of X-ray (unit: cps);

FIG. 6 shows the results of analyses by X-ray diffractometry of the ironsulfide powder prepared in Example 8 and of this powder which hadundergone a one day durability test at 70° C. and 70% RH, in which Xaxis (abscissa) indicates angle of diffraction 2θ (degree) and Y axis(ordinate) indicates the intensity of X-ray (unit: cps);

FIG. 7 shows the results of analyses by X-ray diffractometry of the ironsulfide powder prepared in Example 9 and of this powder which hadundergone a one day durability test at 70° C. and 70% RH, in which Xaxis (abscissa) indicates angle of diffraction 2θ (degree) and Y axis(ordinate) indicates the intensity of X-ray (unit: cps); and

FIG. 8 shows the results of analyses by X-ray diffractometry of the ironsulfide powders prepared in Example 8, Example 12, Example 13, Example14 and Example 15, in which X axis (abscissa) indicates angle ofdiffraction 2θ (degree) and Y axis (ordinate) indicates the intensity ofX-ray (unit: cps);

DESIGNATION OF THE REFERENCE NUMERALS

1: X-ray diffraction pattern of the iron sulfide powder prepared inExample 1.

2: X-ray diffraction pattern of the iron sulfide powder prepared inExample 1 which had undergone a durability test.

3: X-ray diffraction pattern of the iron sulfide powder prepared inExample 1.

4: X-ray diffraction pattern of the iron sulfide powder prepared inExample 2.

5: X-ray diffraction pattern of the iron sulfide powder prepared inExample 3.

6: X-ray diffraction pattern of the iron sulfide powder prepared inComparative Example 1.

7: X-ray diffraction pattern of the iron sulfide powder prepared inComparative Example 1 which had undergone a durability test.

8: X-ray diffraction pattern of the iron sulfide powder prepared inComparative Example 3.

9: X-ray diffraction pattern of the iron sulfide powder prepared inComparative Example 3 which had undergone a durability test.

10: X-ray diffraction pattern of the iron sulfide powder prepared inExample 5.

11: X-ray diffraction pattern of the iron sulfide powder prepared inExample 5 which had undergone a durability test.

12: X-ray diffraction pattern of the iron sulfide powder prepared inExample 8.

13: X-ray diffraction pattern of the iron sulfide powder prepared inExample 8 which had undergone a durability test.

14: X-ray diffraction pattern of the iron sulfide powder prepared inExample 9.

15: X-ray diffraction pattern of the iron sulfide powder prepared inExample 9 which had undergone a durability test.

16: X-ray diffraction pattern of the iron sulfide powder prepared inExample 14.

17: X-ray diffraction pattern of the iron sulfide powder prepared inExample 13.

18: X-ray diffraction pattern of the iron sulfide powder prepared inExample 12.

19: X-ray diffraction pattern of the iron sulfide powder prepared inExample 8.

20: X-ray diffraction pattern of the iron sulfide powder prepared inExample 15.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained in detail below. It is essential thatthe iron sulfides of the invention should contain FeM_(x)N_(y)S_(z)(wherein M represents an alkaline earth metal, N represents an alkalimetal, and x, y, and z, indicating the molar proportions of therespective elements, represent numbers satisfying 0.01<x≦0.5, y≦0.2, and0.7≦z≦1.4) as an essential component.

Iron sulfides are generally known to be non-stoichiometric with respectto the molar proportions of iron and sulfur. Namely, the molar ratio ofsulfur to iron can be any value around 1. Like the mackinawite ironsulfides which have been used hitherto, the iron sulfides of theinvention have a sulfur/iron molar ratio z of from 0.7 to 1.4. From thestandpoint of obtaining a more stable mackinawite structure, the molarratio z is preferably from 0.8 to 1.0.

In the iron sulfides of the invention, the amount of the alkaline earthmetal is such that the alkaline earth metal/iron molar ratio x is in therange of 0.01<x≦0.5. In case where the molar ratio x is 0.01 or lower,an improvement in durability is not expected. On the other hand, in casewhere x exceeds 0.5, the resultant effect of improving stability islower than in the case where x is within that range.

In preparing iron sulfides having the composition according to theinvention, i.e., containing an alkaline earth metal in the specificamount, the iron sulfides obtained have different XRD patterns dependingon the sequence of addition of ingredients including the alkaline earthmetal. Specifically, an iron sulfide prepared by mixing a salt ofbivalent iron with an aqueous solution containing sulfur ions,subsequently adding an alkaline earth metal ingredient thereto, andadjusting the pH of the mixture to 7 or higher shows an XRD patterncharacteristic of mackinawite structures, while an iron sulfide preparedby mixing a salt of bivalent iron with an aqueous solution containingsulfur ions in the presence of an alkaline earth metal ingredient andadjusting the pH of the mixture to 7 or higher has a denaturedmackinawite structure, which will be described later. The presentinventors have found that in either case, no peak attributable to thealkaline earth metal ingredient is observed as long as the iron sulfidehas a composition within the range according to the invention, and thatthe effect of improving durability is exceedingly high when thecomposition is within that range. In case where an alkaline earth metalingredient is added in an amount exceeding the upper limit of thatrange, peaks attributable to a compound of the alkaline earth metalingredient are observed and the resultant durability-improving effect isalmost equal to that of compositions containing the alkaline earth metalin a lower proportion. It is, however, noted that the range of x optimalfor the durability-improving effect varies slightly with the kind of thealkaline earth metal. The optimal ranges for magnesium, calcium,strontium and barium are 0.04≦x≦0.5, 0.03≦x≦0.4, 0.02≦x≦0.3 and0.01<x≦0.2, respectively.

Any desired combination of two or more of those four alkaline earthmetal elements can be used in the invention. The most preferred of thosefour alkaline earth metal elements in the invention are calcium and/ormagnesium. Calcium and magnesium not only are readily incorporated intoiron sulfides, but also are industrially easily available at low costunlike strontium and pose no problem concerning toxicity unlike barium.

In the iron sulfides of the invention, the upper limit of the amount ofthe alkali metal in terms of alkali metal/iron molar ratio y should belower than in the case of the mackinawite iron sulfides which have beenknown. Specifically, it is essential that the molar ratio y should be0.2 or lower. In case where the amount of the alkali metal is largerthan 0.2, it is difficult to obtain a highly durable iron sulfide, whichis an aim of the invention. The smaller the amount of the alkali metal,the more the durability can be expected to be improved. The amount ofthe alkali metal is more preferably 0.1 or smaller. The iron sulfides ofthe invention can be ones which contain no alkali metal. For example, aniron sulfide in which y is 0 can be obtained according to the inventionby producing it from starting materials containing no alkali metal or byremoving an alkali metal by washing, etc. The kind of the alkali metalvaries depending on the starting materials used. For example, in thecase where sodium sulfide or sodium hydrosulfide, which are industriallyeasily available, is used as a material for sulfur ions, N in theempirical formula is sodium.

In the invention, durability was evaluated at various temperatures andrelative humidities. Mackinawite iron sulfides synthesized by theprocess used hitherto, when placed in a summer atmosphere having atemperature of up to 30° C. and a relative humidity of up to 70%, cometo decompose in one day to show an XRD pattern bearing peaksattributable to sulfur. In 7 days, these iron sulfides completelydecompose and the peaks attributable to a mackinawite structuredisappear. When these iron sulfides are allowed to stand under theconditions of a temperature of 50° C. and a relative humidity of 70%,they completely decompose in one day and the peaks attributable to amackinawite structure disappear. In contrast, the iron sulfides of theinvention are characterized by having highly improved durability. Evenwhen the iron sulfides of the invention are subjected to a one-daydurability test under the severer conditions of a temperature of 70° C.and a relative humidity of 70%, the degree of remanence is 50% or higher(the degree of decomposition is lower than 50%).

The reason why the addition of an alkaline earth metal improvesdurability has not been elucidated. It is, however, presumed that thealkaline earth metal is incorporated into the iron sulfide and a layerwhich prevents oxidative decomposition is formed on the surface of theiron sulfide or that the alkaline earth metal serves to prevent theprogress of oxidative decomposition.

It is further presumed that when an alkaline earth metal is incorporatedinto an iron sulfide, it does not replace iron in the iron sulfide butis incorporated in the form of the hydroxide or oxide of the alkalineearth metal.

The term “mackinawite structure” as used herein means the structureshown in JCPDS card (Powder Diffraction File), 15-37. The XRD patternthereof is as shown in the following Table 1.

TABLE 1 Spacing (A) I/I1 hk1 5.03 100  001 2.97 80 101 2.60  5 110 2.3180 111 1.832 40 200 1.808 80 112 1.725 40 201 1.674 20 003 1.562 30 2111.524 20 103

The term “denatured mackinawite structure” as used herein means astructure characterized by showing the XRD pattern of mackinawitestructure shown in Table 1 above in which the spacing between the 001planes has increased in the c axis direction to a value of from 5.03 Ato 5.53 A and the ratio of the intensity for the diffraction peakattributable to any other hkl planes to that for the diffraction peakattributable to the 001 planes is 20/100 or lower.

Although the reason why a denatured mackinawite structure is formed hasnot been elucidated, it is presumed that when an alkaline earth metalingredient is present during the generation of an iron sulfide, thealkaline earth metal is incorporated into the iron sulfide in such amanner that it elongates the c axis, i.e., increases the distancebetween each iron atom layer and the adjacent sulfur atom layers, withdisordering the configuration of iron atoms and sulfur atoms along the aaxis and b axis.

Iron sulfides having the denatured mackinawite structure described abovewere investigated, and as a result, the following was found. When thiskind of iron sulfide is treated by dispersing it in water, adding a weakacid such as acetic acid thereto, and then recovering the iron sulfideby filtration and washing it, then part of the alkaline earth metal islost by dissolution and the iron sulfide recovered shows an XRD patterncharacteristics of the mackinawite structure. Namely, it was found thatan iron sulfide of the denatured mackinawite structure reversiblychanges into an iron sulfide of a mackinawite structure. Consequently,the denatured mackinawite structure according to the invention isthought to be a special form of the mackinawite structure which has beenknown.

Processes for producing the iron sulfides of the invention will beexplained in detail below.

One process of the invention comprises mixing an aqueous solution of asalt of bivalent iron, an aqueous solution containing sulfur ions and analkaline earth metal ingredient and adjusting the pH of the resultantslurry to 7.0 or higher to thereby obtain a mackinawite iron sulfidehaving excellent durability.

In the process of the invention, the sequence of mixing the threeingredients can be suitably selected. Namely, the following method maybe employed: a method comprising mixing an aqueous solution of a salt ofbivalent iron with an aqueous solution containing sulfur ions and thenmixing the resultant mixture with an alkaline earth metal ingredient; amethod comprising mixing an aqueous solution of a salt of bivalent ironwith an alkaline earth metal ingredient and then mixing the resultantmixture with an aqueous solution containing sulfur ions; or a methodcomprising mixing an aqueous solution containing sulfur ions with analkaline earth metal ingredient and then mixing the resultant mixturewith an aqueous solution of a salt of bivalent iron. A method which isoptimal for the production process can be selected.

The present inventors have further found that in the case where analkaline earth metal ingredient is present when an iron sulfide isformed by mixing an aqueous solution of a salt of bivalent iron with anaqueous solution containing sulfur ions, the iron sulfide yielded hasnot a mackinawite structure but a denatured mackinawite structure.

The salt of bivalent iron used as an aqueous solution in the processesof the invention is not particularly limited as long as it is awater-soluble salt of bivalent iron. Examples thereof include iron(II)chloride, iron(II) nitrate, iron(II) sulfate, and iron(II) acetate. Mostpreferred of these is iron(II) chloride because it is industriallyeasily available and is inexpensive. A solution obtained by dissolvingscrap iron or the like in hydrochloric acid or a waste liquid resultingfrom the washing of sheet or plate iron with hydrochloric acid can alsobe advantageously used. On the other hand, iron(II) sulfate is notpreferred because it is apt to form a poorly soluble salt with thealkaline earth metal ingredient when this ingredient is calcium,strontium or barium, making it necessary to carefully and slowly mixstarting materials. However, in the case of using magnesium as thealkaline earth metal ingredient, iron(II) sulfate can be advantageouslyused.

The concentration of the aqueous solution of a bivalent iron salt usedis not particularly limited. However, too low concentrations may beindustrially disadvantageous because of the necessity of a reactionvessel having a large capacity and an increase in the amount of theslurry to be filtered. Conversely, too high concentrations not only areapt to arouse troubles such as salt precipitation by temperaturefluctuations but also may result in a slurry which has an increasedviscosity and is difficult to handle or evenly mix. Specifically, thepreferred range of the concentration of the salt of bivalent iron canbe, for example, from 1 to 25 wt %, more preferably from 3 to 20 wt %,in terms of iron concentration.

The solution containing sulfur ions used in the processes of theinvention is not particularly limited as long as it contains sulfurions. Any solution prepared by dissolving the sulfide or hydrosulfide ofan alkali metal salt, sulfide or hydrosulfide of an ammonium salt, orsulfide or hydrosulfide of an alkaline earth metal in water can beadvantageously used. Of these sulfur compounds, sodium sulfide andsodium hydrosulfide are most preferably used because those areindustrially easily available and are inexpensive. A solution preparedby causing an aqueous solution of an alkali, e.g., sodium hydroxide, toabsorb the hydrogen sulfide obtained in a petroleum desulfurization stepcan also be advantageously used.

The concentration of the sulfur ion-containing solution to be used isnot particularly limited. However, too high concentrations and too lowconcentrations are undesirable for the same reasons as in the case ofthe aqueous solution of a bivalent iron salt. Specifically, thepreferred range of the concentration of sulfur ions can be, for example,from 1 to 15 wt %, more preferably from 2 to 10 wt %, in terms of sulfurconcentration.

The alkaline earth metal ingredient is not particularly limited as longas it is water-soluble. Furthermore, an alkaline earth metal ingredientwhich is water-insoluble, such as carbonates, can be used as long as itdissolves in acid solutions. Examples of the alkaline earth metalingredient include the chlorides, carboxylates, nitrates, hydroxides andsulfides of alkaline earth metals.

The ratio in which the aqueous solution of a salt of bivalent iron andthe aqueous solution containing sulfur ions are mixed with each other isnot particularly limited. However, too high proportions of sulfur ionsnot only result in an increased material cost but also may arouse aproblem that the sulfur ions partly remain unreacted in the motherliquor, resulting in the necessity of wastewater treatment. Conversely,too low proportions of sulfur ions arouse a problem that the excess ironprecipitates as iron hydroxide to yield a mixture of an iron sulfide andthe hydroxide. Although this iron sulfide retains intact properties, theiron sulfide has been diluted by the iron hydroxide and, hence, appearsto have reduced properties. It is therefore preferred to mix the twosolutions in such a proportion that the iron/sulfur molar ratio is inthe range of from 1/0.7 to 1/1.8, more preferably from 1/0.8 to 1/1.5.

The ratio in which the aqueous solution of a salt of bivalent iron andthe alkaline earth metal ingredient are mixed with each other variesdepending on the kind of the alkaline earth metal. However, in casewhere the amount of the alkaline earth metal is too small, a mackinawiteiron sulfide having excellent durability according to the inventioncannot be obtained. Consequently, in the case where the alkaline earthmetal ingredient is magnesium (Mg), it is preferred to mix the twoingredients in such a proportion that the Fe/Mg molar ratio is 1/0.04 orsmaller. In the case where the alkaline earth metal ingredient iscalcium (Ca), the two ingredients are preferably mixed in such aproportion that the Fe/Ca molar ratio is 1/0.03 or smaller. In the casewhere the alkaline earth metal ingredient is strontium (Sr), the twoingredients are preferably mixed in such a proportion that the Fe/Srmolar ratio is 1/0.02 or smaller. In the case where the alkaline earthmetal ingredient is barium (Ba), the two ingredients are, preferablymixed in such a proportion that the Fe/Ba molar ratio is 1/0.01 orsmaller.

Methods for the mixing are not particularly limited, and the semi-batchprocess or continuous process known in chemical engineering can be used.

The temperature at which the ingredients are mixed is not particularlylimited. There is no need of cooling or heating. For example, atemperature of from 10 to 60° C. can be used.

The stirring to be conducted for the mixing is not particularly limited.Any degree of stirring may be used without arousing particular problems,as long as the slurry containing the iron sulfide yielded does notstagnate.

The rate at which the feed materials are mixed is not particularlylimited. However, too low mixing rates may result in loweredproductivity, while too high mixing rates may result in local stagnationor a viscosity increase. Consequently, in the case of a semi-batchprocess, for example, a rate of addition at which all the feed materialsare mixed in a period of from 1 to 240 minutes, preferably from 3 to 120minutes, may be selected. In the case of a continuous process, amaterial feed rate may be selected so that the average residence time isfrom 10 to 240 minutes, preferably from 15 to 120 minutes.

At the time when the mixing described above is completed, a slurrycontaining an iron sulfide of the invention is obtained. In theinvention, it is essential that the slurry pH be 7.0 or higher at thetime when the mixing has been completed. Although an alkali source maybe added after completion of the mixing in order to adjust the slurry pHto 7.0 or higher, it is more preferred to add an alkali sourcebeforehand to the aqueous solution containing sulfur ions. The alkalisource is not particularly limited, and examples thereof include thehydroxides of alkali metals and the hydroxides of alkaline earth metals.Specific examples thereof include sodium hydroxide, potassium hydroxide,calcium hydroxide and barium hydroxide. In case where the slurry has apH lower than 7.0, the alkaline earth metal is not incorporated into theiron sulfide, so that an iron sulfide of the invention cannot beobtained. The higher the slurry pH, the more preferred. Specifically,the slurry pH is adjusted to preferably 8.0 or higher, more preferably10.0 or higher, most preferably around 12.0.

After completion of the mixing of the feed materials, aging may beconducted by continuing the stirring of the resultant slurry so as tokeep the whole slurry homogeneous. The period of aging is notparticularly limited, and the aging may be conducted, for example, for aperiod of from 0 to 300 minutes.

By the method described above, a slurry containing an iron sulfide ofthe invention is obtained. Although this slurry may be used as it is forthe treatment of heavy metals, use thereof not only results in anincreased transportation cost but may arouse the following problem. Whenthis iron sulfide is added in a large amount to treat a fly ash or soilcontaining heavy metals in a high concentration, the waste thus treatedhas too high a water content and is hence difficult to handlethereafter. Because of these drawbacks, the slurry is usually convertedto an iron sulfide powder through filtration, washing, and subsequentdrying.

For the filtration and washing, known methods can be used. However, incase where filtration and washing are insufficient, the alkali metalremaining in the iron sulfide makes it difficult to obtain an ironsulfide having excellent durability. Consequently, the filtration andwashing should be conducted under such conditions that the iron sulfideto be obtained through the filtration and washing has an alkali metalcontent of 0.2 mol or smaller, preferably 0.1 mol or smaller, per moleof the iron.

The drying may be conducted by any known method. However, it is morepreferred to conduct drying in an inert gas atmosphere or in vacuum fromthe standpoint of preventing the iron sulfide from oxidizing.

An iron sulfide of the invention can be obtained by the method describedabove.

As a result of further investigations, it has been found that by addingany of various alkaline earth metal compounds to an iron sulfide havingthe mixture according to the invention, an iron sulfide mixture isobtained which can be more advantageously used as a heavy metal treatingagent; to provide the iron sulfide mixture is one of the objects of theinvention. This iron sulfide mixture can be prepared by preparing aniron sulfide having the component according to the invention,subsequently adding an alkaline earth metal compound thereto, and mixingthe resultant mixture, for example, mechanically with a ball mill or thelike. Alternatively, the iron sulfide mixture can be prepared bypreparing an iron sulfide using an alkaline earth metal ingredientexcessively in an amount larger than the upper limit according to thecomposition of an iron sulfide according to the invention.

Examples of the alkaline earth metal compound which is added or is usedexcessively include the hydroxides, carboxylates, phosphates, sulfites,sulfates and carbonates of alkaline earth metals. Specific examplesthereof include magnesium hydroxide, calcium hydroxide, strontiumhydroxide, barium hydroxide, the corresponding alkaline earth metalsalts of acetic acid, formic acid, oxalic acid, citric acid, stearicacid, etc., the corresponding alkaline earth metal salts of phosphoricacid and polyphosphoric acids, such as calcium hydrogen phosphate, thecarbonates of alkaline earth metals, such as magnesium carbonate andcalcium carbonate, and the sulfates of alkaline earth metals, such asmagnesium sulfate, calcium sulfate, and barium sulfate. Examples thereoffurther include alkaline earth metal salts of sulfurous acid andascorbic acid, which have an antioxidant action, etc.

Those alkaline earth metal compounds do not decrease the durability ofthe iron sulfide of the invention, and have various functions accordingto the kinds thereof. For example, those compounds function to improvethe activity of the iron sulfide in treating heavy metals based on theiron sulfide-supporting effect thereof and to improve durability. Theyfurther function as an aid in heavy metal treatment and to improvehandleability. One or more alkaline earth metal salts may be added tothe iron sulfide of the invention according to purposes. For example,when an acid fly ash is to be treated, it is preferred to add an alkalias an aid. In this case, a mixture of an iron sulfide of the inventionand the hydroxide of an alkaline earth metal can be advantageously used.In the case where the waste to be treated contains a large amount of anoxidizing ingredient such as hexavalent chromium, a mixture of an ironsulfide of the invention and a sulfurous acid salt can be advantageouslyused. It has been found that although the iron sulfides of the inventionhave excellent durability, addition of a carboxylic acid salt,especially the acetate, of an alkaline earth metal further improves thedurability. Such salts can be added for storage under severe conditions.

The amount of such alkaline earth metal compounds to be added is notparticularly limited and varies depending on purposes. For example,those compounds can be added in an amount of from 0.1 to 100 parts byweight per 100 parts by weight of the iron sulfide of the invention.

The iron sulfides and the mixture comprising iron sulfide of theinvention have exceedingly high durability, and a “heavy metal treatingagent” comprising any of those as an effective component has exceedinglyhigh performance. The heavy metal treating agent of the invention willbe explained below in detail.

Examples of heavy metals which can be treated with the heavy metaltreating agent of the invention include lead, cadmium, chromium,mercury, arsenic, selenium, copper, nickel, antimony and molybdenum. Itis a matter of course that the treating agent of the invention can beused not only in the treatment of a waste containing any one of thoseheavy metals but in the treatment of a waste containing any two or moreof those elements.

The heavy metal treating agent of the invention is extremely useful fortreating refuse incineration ashes and fly ashes containing heavymetals. In refuse incineration ashes and fly ashes, the heavy metalswhich were contained in various refuses have been concentrated. Inparticular, this concentration is considerable in fly ashes and fusedfly ashes, and many fused fly ashes contain heavy metals, such as lead,in an amount on the order of percent and should be treated so as to bemade harmless. The fly ashes and fused fly ashes are of several kindssuch as, e.g., alkaline fly ashes, neutral fly ashes, alkaline fused flyashes, and neutral fused fly ashes depending on differences in thestructure of incinerators and in the method of operation thereof.Furthermore, it is known that the kind and content of heavy metalscontained in such a fly ash vary considerably depending on the kind ofthe refuse incinerated. However, the heavy metal treating agent of theinvention can be applied to any kind of fly ash. The heavy metaltreating agent of the invention and water are added to any of thoserefuse incineration ashes and fly ashes and the resulting mixture iskneaded.

The amount of the heavy metal treating agent of the invention to beadded cannot be fixed unconditionally because it varies depending on thekind and total amount of the heavy metals contained in the refuseincineration ash or fly ash to be treated. For example, the amountthereof is generally from 0.1 to 50 wt %, preferably from 0.5 to 30 wt%, based on the weight of the refuse incineration ash or fly ash. It isdesirable that the refuse incineration ash or fly ash be sampledbeforehand to determine the minimum addition amount through a laboratorytest and further determine the optimal addition amount while takingaccount of fluctuations of the amount of heavy metals contained in therefuse incineration ash or fly ash. Even though the treating agent isadded excessively, no problems arise because the mercury (Hg), forexample, becomes not a soluble substance but a polysulfide.

The amount of water added varies depending on the nature of the refuseincineration ash or fly ash. For example, however, the amount thereof isusually from 10 to 40 wt % based on the weight of the refuseincineration ash or fly ash. Methods for kneading and the kneadingperiod are not particularly limited, and the kneading can be conductedby a known method. By the treatment, the soluble heavy metals areconverted to insoluble sulfides or iron salts.

The heavy metal treating agent of the invention is effective also in thetreatment of a soil containing heavy metals. The heavy metal treatingagent is added to the soil containing heavy metals, optionally togetherwith water, and the resultant mixture is kneaded.

The amount of the heavy-metal treating agent of the invention to beadded cannot be fixed unconditionally because it varies depending on thetotal amount of the heavy metals contained in the soil. For example, theamount thereof is from 0.1 to 20 wt % based on the weight of the soil tobe treated. It is desirable that the soil be sampled beforehand todetermine the minimum addition amount through a laboratory test so as toadd the treating agent in slight excess for safety. In the case wherethe amount of water contained in the soil is small, water may beoptionally added to the soil so as to result in a water content in thesoil of usually from 10 to 60 wt %, although it varies depending on thekind of the soil. Methods for kneading and the kneading period are notparticularly limited, and the kneading can be conducted by a knownmethod. By the treatment, the soluble heavy metals are converted toinsoluble sulfides or iron salts.

Furthermore, the heavy metal treating agent of the invention can be usedto treat a wastewater containing heavy metals. The heavy metal treatingagent is added to the wastewater containing heavy metals and theresultant mixture is stirred. The amount of the heavy metal treatingagent added cannot be fixed unconditionally because it varies dependingon the total amount of the heavy metals contained in the wastewater. Itis desirable that the wastewater be sampled beforehand to determine theminimum addition amount through a laboratory test so as to add thetreating agent in slight excess for safety. In this treatment, when thewastewater has a low pH, the iron sulfide may decompose to generatehydrogen sulfide. It is therefore preferred to adjust the pH of thewastewater beforehand. In this case, the pH of the wastewater isadjusted to 3.0 or higher, preferably 6.0 or higher. Methods forstirring and the stirring period are not particularly limited, and thestirring can be conducted by a known method. By the treatment, the heavymetals contained in the wastewater are converted to insoluble sulfidesor iron salts. The treating agent can be used in combination with, forexample, an inorganic coagulant/precipitant usually used incoagulation/precipitation treatments, such as ferric chloride,poly(aluminum chloride), or aluminum sulfate, or with a polymericcoagulant which accelerates coagulation.

The invention will be described in more detail by reference to thefollowing Examples, but the invention should not be construed as beinglimited to these Examples only. The methods for determination used inthe Examples are as follows.

(1) Methods for Determining Chemical Composition

Iron, sulfur, calcium, strontium and barium were determined with afluorescent X-ray spectrometer (Type JSX-3200, manufactured by JEOLLtd.). Sodium, magnesium and iron were determined by dissolving thesample in hydrochloric acid and analyzing the solution with an ICPemission spectrometer (Type Optima 3000, manufactured by Perkin ElmerCorp.). After the determinations, the molar proportion of each of thealkaline earth metal element, alkali metal element, and sulfur to theiron was determined.

(2) Method for Determining Crystal Structure

Determination was made with an X-ray diffractometer (Type MXP-3,manufactured by Mac Science; copper target).

(3) Proportion of Residual Iron Sulfide

The proportion was calculated using the following equation:Proportion of residual iron sulfide (%)=I2/I1×100wherein I1 is the main peak intensity for the iron sulfide before adurability test and I2 is the main peak intensity for the iron sulfideafter the durability test.

EXAMPLE 1

An aqueous iron(II) chloride solution (0.5 mol/liter) and an aqueoussodium sulfide solution (1.0 mol/liter) were continuously introducedwith stirring into a stainless-steel vessel for continuous reactionhaving an effective capacity of 750 ml at rates of 500 ml/hr and 250ml/hr, respectively, while keeping the vessel at 25° C. Thus, an ironsulfide slurry was prepared.

Into a glass reactor having a capacity of 2 liters was introduced 1,000ml of the iron sulfide slurry obtained. Thereto was added 83 mmol ofcalcium chloride with stirring. Furthermore, 48% aqueous NaOH solutionwas added to adjust the slurry pH to 12.8. This slurry was then aged bycontinuously stirring it for 60 minutes. The slurry obtained after theaging was filtered, and the obtained cake was washed, dried, and thenpulverized to obtain an iron sulfide powder. X-ray diffractometryrevealed that the iron sulfide powder obtained had a mackinawitestructure. The X-ray diffraction chart obtained is shown in FIGS. 1 and2. Analysis for composition determination revealed that the molarproportions of the respective components were represented byFeCa_(0.22)Na_(0.016)S_(1.00).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a mackinawite structure and no peakattributable to sulfur or iron(III) oxide, which are products ofdecomposition of iron sulfide, was observed. The X-ray diffraction chartobtained is shown in FIG. 1. The proportion of the residual iron sulfidewas calculated from the ratio between the main peak intensity for theiron sulfide before the durability test and that for the iron sulfideafter the test, and was found to be 85%.

EXAMPLE 2

An iron sulfide powder was obtained by conducting completely the sameprocedure as in Example 1, except that the slurry pH was adjusted to11.7. X-ray diffractometry revealed that the iron sulfide powderobtained had a mackinawite structure. The X-ray diffraction chartobtained is shown in FIG. 2. Analysis for composition determinationrevealed that the molar proportions of the respective components wererepresented by FeCa_(0.073)Na_(0.032)S_(0.99).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a mackinawite structure and slightpeaks attributable to sulfur, which is a product of decomposition ofiron sulfide, were observed. The proportion of the residual iron sulfidewas calculated from the ratio between the main peak intensity for theiron sulfide before the durability test and that for the iron sulfideafter the test, and was found to be 71%.

EXAMPLE 3

An iron sulfide powder was obtained by conducting completely the sameprocedure as in Example 1, except that the slurry pH was adjusted to10.3. X-ray diffractometry revealed that the iron sulfide powderobtained had a mackinawite structure. The X-ray diffraction chartobtained is shown in FIG. 2. Analysis for composition determinationrevealed that the molar proportions of the respective components wererepresented by FeCa_(0.035)Na_(0.077)S_(0.99).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. As a result of the analysis by X-ray diffractometry ofthe powder which had undergone the durability test, peaks attributableto sulfur, which is a product of decomposition of iron sulfide, wereobserved besides peaks attributable to mackinawite iron sulfide. Theproportion of the residual iron sulfide was calculated from the ratiobetween the main peak intensity for the iron sulfide before thedurability test and that for the iron sulfide after the test, and wasfound to be 53%.

EXAMPLE 4

An iron sulfide powder was obtained by conducting completely the sameprocedure as in Example 1, except that the amount of calcium chlorideadded was changed to 220 mmol and the slurry pH was adjusted to 12.6.The iron sulfide powder obtained was analyzed by X-ray diffractometry.As a result, peaks attributable to calcium hydroxide were observedbesides peaks attributable to mackinawite iron sulfide. Analysis forcomposition determination revealed that the molar proportions of therespective components were represented by FeCa_(0.43)Na_(0.004)S_(0.98).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. As a result of the analysis by X-ray diffractometry ofthe powder which had undergone the durability test, peaks attributableto calcium hydroxide and calcium carbonate were observed besides peaksattributable to mackinawite iron sulfide. The proportion of the residualiron sulfide was calculated from the ratio between the main peakintensity for the iron sulfide before the durability test and that forthe iron sulfide after the test, and was found to be 87%.

COMPARATIVE EXAMPLE 1

An iron sulfide slurry continuously prepared by the same method as inExample 1 was filtered, and the obtained cake was washed, dried, andthen pulverized to obtain an iron sulfide powder. X-ray diffractometryrevealed that the iron sulfide powder obtained had a mackinawitestructure. The X-ray diffraction chart obtained is shown in FIG. 3.Analysis for composition determination revealed that the molarproportions of the respective components were represented byFeNa_(0.22)S_(0.99).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test was a mixture of sulfur and iron(III)oxide, which are products of decomposition of iron sulfide, and no peakattributable to an iron sulfide was observed. The X-ray diffractionchart obtained is shown in FIG. 3.

COMPARATIVE EXAMPLE 2

To 30 g of the iron sulfide powder prepared in Comparative Example 1 wasadded 4 g of calcium hydroxide. This mixture was treated with a ballmill for 30 minutes to prepare an iron oxide/calcium hydroxide mixturecomposition. X-ray diffractometry revealed that this composition was amixture of mackinawite iron sulfide and calcium hydroxide. Analysis forcomposition determination revealed that the molar proportions of therespective components were represented by FeCa_(0.19)Na_(0.22)S_(0.99).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test was a mixture of sulfur and iron(III)oxide, which are products of decomposition of iron sulfide, and ofcalcium hydroxide and calcium carbonate. No peak attributable to an ironsulfide was observed.

COMPARATIVE EXAMPLE 3

Commercial iron(II) sulfide lump of reagent grade was pulverized toobtain an iron sulfide powder. X-ray diffractometry revealed that theiron sulfide powder obtained had a pyrrhotite structure. The X-raydiffraction chart obtained is shown in FIG. 4.

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. As a result of the analysis by X-ray diffractometry ofthe powder which had undergone the durability test, peaks attributableto a pyrrhotite structure and to sulfur, which is a product ofdecomposition of iron sulfide, were observed. The X-ray diffractionchart obtained is shown in FIG. 4.

The results of Example 1 and Comparative Examples 1, 2 and 3 show thatthe iron sulfide according to the invention had far higher durabilitythan the known synthetic iron sulfides. In addition, the calcium addedis judged not to have been present as a mere mixture but to have beenincorporated into the iron sulfide. The results of Examples 1 to 4 showthat the higher the slurry pH, the more the calcium was apt to beincorporated into the iron sulfide, and that the larger the amount ofcalcium incorporated, the better the stability of the iron sulfide. Theresults further show that the amount of an alkaline earth metal whichcan be incorporated into an iron oxide has an upper limit, which ispresumed to be around 0.4 in the case of calcium, and that addition ofcalcium in an amount larger than the upper limit results in an ironsulfide/calcium hydroxide mixture composition. From these, it can bejudged that in the case where the alkaline earth metal is calcium, aniron sulfide having improved durability of the invention is obtainedwhen the calcium/iron molar ratio is in the range of from 0.03 to 0.4.In FIG. 2, a diffraction peak for the iron sulfide obtained in Example 1has shifted to the smaller angle side as compared with the correspondingpeaks for the iron sulfides obtained in Examples 2 and 3. It is thoughtthat this shift is due to the incorporation of a larger amount ofcalcium in the iron sulfide.

EXAMPLE 5

An iron sulfide slurry was continuously prepared by the same method asin Example 1. Into a glass reactor having a capacity of 2 L wasintroduced 1,000 ml of the iron sulfide slurry obtained. Thereto wasadded 83 mmol of barium chloride with stirring. Furthermore, 48% aqueousNaOH solution was added to adjust the slurry pH to 13.0. This slurry wasthen aged by continuously stirring it for 60 minutes. The slurryobtained after the aging was filtered, and the obtained cake was washed,dried, and then pulverized to obtain an iron sulfide powder. X-raydiffractometry revealed that the iron sulfide powder obtained had amackinawite structure. The X-ray diffraction chart obtained is shown inFIG. 5. Analysis for composition determination revealed that the molarproportions of the respective components were represented byFeBa_(0.078)Na_(0.014)S_(0.96).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. The powder which had undergone the durability test wasfound to have a mackinawite structure and no peak attributable to sulfuror iron(III) oxide, which are products of decomposition of iron sulfide,was observed. The X-ray diffraction chart obtained is shown in FIG. 5.The proportion of the residual iron sulfide was calculated from theratio between the main peak intensity for the iron sulfide before thedurability test and that for the iron sulfide after the test, and wasfound to be 93%.

EXAMPLE 6

An iron sulfide slurry was continuously prepared by the same method asin Example 1. Into a glass reactor having a capacity of 2 liters wasintroduced 1,000 ml of the iron sulfide slurry obtained. Thereto wasadded 66 mmol of strontium chloride with stirring. Furthermore, 48%aqueous NaOH solution was added to adjust the slurry pH to 13.1. Thisslurry was then aged by continuously stirring it for 60 minutes. Theslurry obtained after the aging was filtered, and the obtained cake waswashed, dried, and then pulverized to obtain an iron sulfide powder.X-ray diffractometry revealed that the iron sulfide powder obtained hada mackinawite structure. Analysis for composition determination revealedthat the molar proportions of the respective components were representedby FeSr_(0.095)Na_(0.023)S_(0.98).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a mackinawite structure and no peakattributable to sulfur or iron(III) oxide was observed. The proportionof the residual iron sulfide was calculated from the ratio between themain peak intensity for the iron sulfide before the durability test andthat for the iron sulfide after the test, and was found to be 88%.

EXAMPLE 7

An iron sulfide slurry was continuously prepared by the same method asin Example 1. Into a glass reactor having a capacity of 2 liters wasintroduced 1,000 ml of the iron sulfide slurry obtained. Thereto wasadded 130 mmol of magnesium chloride with stirring. Furthermore, 48%aqueous NaOH solution was added to adjust the slurry pH to 12.9. Thisslurry was then aged by continuously stirring it for 60 minutes. Theslurry obtained after the aging was filtered, and the obtained waswashed, dried, and then pulverized to obtain an iron sulfide powder.X-ray diffractometry revealed that the iron sulfide powder obtained hada mackinawite structure. Analysis for composition determination revealedthat the molar proportions of the respective components were representedby FeMg_(0.37)Na_(0.068)S_(0.95).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a mackinawite structure and lightpeaks attributable to sulfur, which is a product of decomposition ofiron sulfide, were observed. The proportion of the residual iron sulfidewas calculated from the ratio between the main peak intensity for theiron sulfide before the durability test and that for the iron sulfideafter the test, and was found to be 72%.

The results of Examples 5 to 7 show that the alkaline earth metals otherthan calcium also functioned to improve the durability of a syntheticiron sulfide.

EXAMPLE 8

Into a glass reactor having a capacity of 2 liters were introduced 480mmol of sodium hydrosulfide of commercial reagent grade, 720 mmol ofsodium hydroxide, and 1,000 g of water. The contents were kept at 25° C.with a water bath with stirring to dissolve the solids. To this solutionwas added during 40 minutes a solution prepared by dissolving 480 mmolof iron(II) chloride and 120 mmol of calcium chloride in 600 g of water.After completion of the addition, pH of the resulting slurry was 12.3.This slurry was then aged by continuously stirring it for 30 minutes.The slurry obtained after the aging was filtered, and the obtained cakewas washed, dried, and then pulverized to obtain an iron sulfide powder.X-ray diffractometry revealed that the iron sulfide powder obtained hada denatured mackinawite structure showing a broad peak at around 17 to18°. The X-ray diffraction chart obtained is shown in FIGS. 6 and 8.Analysis for composition determination revealed that the molarproportions of the respective components were represented byFeCa_(0.16)Na_(0.006)S_(0.90).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a denatured mackinawite structureshowing a broad peak at around 17 to 18°, and no peak attributable tosulfur or iron(III) oxide, which are products of decomposition of ironsulfide, was observed. The X-ray diffraction chart obtained is shown inFIG. 6. The proportion of the residual iron sulfide was calculated fromthe ratio between the main peak intensity for the iron sulfide beforethe durability test and that for the iron sulfide after the test, andwas found to be 91%.

Furthermore, part of the iron sulfide powder obtained was subjected to a3 months indoor storage test from summer to autumn. X-ray diffractometryrevealed that the powder which had undergone the storage test had adenatured mackinawite structure and no peak attributable to sulfur oriron(III) oxide, which are products of decomposition of iron sulfide,was observed.

EXAMPLE 9

Into a glass reactor having a capacity of 2 liters were introduced 480mmol of sodium hydrosulfide of commercial reagent grade, 720 mmol ofsodium hydroxide, and 1,000 g of water. The contents were kept at 25° C.with a water bath with stirring to dissolve the solids. To this solutionwas added during 40 minutes a solution prepared by dissolving 480 mmolof iron(II) chloride and 120 mmol of barium chloride in 600 g of water.After completion of the addition, pH of the resulting slurry was 12.6.This slurry was then aged by continuously stirring it for 30 minutes.The slurry obtained after the aging was filtered, and the solid takenout was washed, dried, and then pulverized to obtain an iron sulfidepowder. X-ray diffractometry revealed that the iron sulfide powderobtained had a denatured mackinawite structure showing a broad peak ataround 17 to 18°. The X-ray diffraction chart obtained is shown in FIG.7. Analysis for composition determination revealed that the molarproportions of the respective components were represented byFeBa_(0.083)Na_(0.028)S_(0.088).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a denatured mackinawite structureshowing a broad peak at around 17 to 18°, and slight peaks attributableto sulfur, which is a product of decomposition of iron sulfide, wereobserved. The X-ray diffraction chart obtained is shown in FIG. 7. Theproportion of the residual iron sulfide was calculated from the ratiobetween the main peak intensity for the iron sulfide before thedurability test and that for the iron sulfide after the test, and wasfound to be 87%.

EXAMPLE 10

Into a glass reactor having a capacity of 2 liters were introduced 480mmol of sodium hydrosulfide of commercial reagent grade, 720 mmol ofsodium hydroxide, and 1,000 g of water. The contents were kept at 25° C.with a water bath with stirring to dissolve the solids. To this solutionwas added during 40 minutes a solution prepared by dissolving 480 mmolof iron(II) chloride and 120 mmol of strontium chloride in 600 g ofwater. After completion of the addition, pH of the resulting slurry was12.4. This slurry was then aged by continuously stirring it for 30minutes. The slurry obtained after the aging was filtered, and the solidtaken out was washed, dried, and then pulverized to obtain an ironsulfide powder. X-ray diffractometry revealed that the iron sulfidepowder obtained had a denatured mackinawite structure showing a broadpeak at around 17 to 18°. Analysis for composition determinationrevealed that the molar proportions of the respective components wererepresented by FeSr_(0.097)Na_(0.019)S_(0.92).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a denatured mackinawite structureshowing a broad peak at around 17 to 18°, and no peak attributable tosulfur or iron(III) oxide, which are products of decomposition of ironsulfide, was observed. The proportion of the iron sulfide was calculatedfrom the ratio between the main peak intensity for the iron sulfidebefore the durability test and that for the iron sulfide after the test,and was found to be 93%.

EXAMPLE 11

Into a glass reactor having a capacity of 2 liters were introduced 420mmol of sodium hydrosulfide of commercial reagent grade, 780 mmol ofsodium hydroxide, and 1,000 g of water. The contents were kept at 25° C.with a water bath with stirring to dissolve the solids. To this solutionwas added during 40 minutes a solution prepared by dissolving 420 mmolof iron(II) sulfate and 180 mmol of magnesium sulfate in 600 g of water.After completion of the addition, pH of the resulting slurry was 10.5.This slurry was then aged by continuously stirring it for 30 minutes.The slurry obtained after the aging was filtered, and the solid takenout was washed, dried, and then pulverized to obtain an iron sulfidepowder. X-ray diffractometry revealed that the iron sulfide powderobtained had a denatured mackinawite structure showing a broad peak ataround 17 to 18°. Analysis for composition determination revealed thatthe molar proportions of the respective components were represented byFeMg_(0.37)Na_(0.079)S_(0.94).

Subsequently, part of the iron sulfide powder obtained was placed in athermo-hygrostatic chamber having a temperature of 70° C. and a relativehumidity of 70% and allowed to stand therein for 1 day to conduct adurability test. X-ray diffractometry revealed that the powder which hadundergone the durability test had a denatured mackinawite structureshowing a broad peak at around 17 to 18°, and slight peaks attributableto sulfur, which is a product of decomposition of iron sulfide, wereobserved. The proportion of the residual iron sulfide was calculatedfrom the ratio between the main peak intensity for the iron sulfidebefore the durability test and that for the iron sulfide after the test,and was found to be 78%.

The results of Examples 8 to 11 show that when an aqueous solution of asalt of bivalent iron was mixed with an aqueous solution containingsulfur ions in the presence of an alkaline earth metal ingredient, thenan iron sulfide having a denatured mackinawite structure was obtained.

EXAMPLE 12

The same procedure as in Example 8 was conducted, except that theamounts of the sodium hydrosulfide, sodium hydroxide, iron(II) chloride,and calcium chloride were changed to 420 mmol, 780 mmol, 420 mmol and180 mmol, respectively.

After completion of the addition, the slurry pH was 12.6. The powderobtained was an iron sulfide having a denatured mackinawite structure,in which the molar proportions of the respective components wererepresented by FeCa_(0.28)Na_(0.002)S_(0.88). The X-ray diffractionchart obtained is shown in FIG. 8. The powder which had undergone thedurability test was an iron sulfide having a denatured mackinawitestructure. The proportion of the residual iron sulfide was 88%.

EXAMPLE 13

The same procedure as in Example 8 was conducted, except that theamounts of the sodium hydrosulfide, sodium hydroxide, iron(II) chloride,and calcium chloride were changed to 360 mmol, 840 mmol, 360 mmol and240 mmol, respectively.

After completion of the addition, the slurry pH was 12.5. The powderobtained was an iron sulfide having a denatured mackinawite structure,and peaks attributable to a trace amount of calcium hydroxide wereobserved. The molar proportions of the respective components wererepresented by FeCa_(0.39)Na_(0.001)S_(0.91). The X-ray diffractionchart obtained is shown in FIG. 8. The powder which had undergone thedurability test was an iron sulfide having a denatured mackinawitestructure, and peaks attributable to trace amounts of calcium hydroxideand calcium carbonate were observed. The proportion of the residual ironsulfide was 93%.

EXAMPLE 14

The same procedure as in Example 8 was conducted, except that theamounts of the sodium hydrosulfide, sodium hydroxide, iron(II) chlorideand calcium chloride were changed to 300 mmol, 900 mmol, 300 mmol and300 mmol, respectively.

After completion of the addition, the slurry pH was 12.5. The powderobtained was a mixture of an iron sulfide having a denatured mackinawitestructure and calcium hydroxide. The molar proportions of the respectivecomponents were represented by FeCa_(0.66)Na_(0.001)S_(0.88). The X-raydiffraction chart obtained is shown in FIG. 8. The powder which hadundergone the durability test was a mixture of an iron sulfide having adenatured mackinawite structure, calcium hydroxide, and calciumcarbonate. The proportion of the residual iron sulfide was 93%.

EXAMPLE 15

The same procedure as in Example 8 was conducted, except that theamounts of the sodium hydrosulfide, sodium hydroxide, iron(II) chlorideand calcium chloride were changed to 540 mmol, 660 mmol, 270 mmol and 60mmol, respectively.

After completion of the addition, the slurry pH was 11.1. The powderobtained was an iron sulfide having a denatured mackinawite structure,in which the molar proportions of the respective components wererepresented by FeCa_(0.038)Na_(0.089)S_(0.85). The X-ray diffractionchart obtained is shown in FIG. 8. The powder which had undergone thedurability test was a mixture of an iron sulfide having a denaturedmackinawite structure and sulfur, which was a product of decompositionthereof. The proportion of the residual iron sulfide was 56%.

COMPARATIVE EXAMPLE 4

The same procedure as in Example 5 was conducted, except that theamounts of the sodium hydrosulfide, sodium hydroxide, iron(II) chlorideand calcium chloride were changed to 600 mmol, 600 mmol, 600 mmol and 10mmol, respectively.

After completion of the addition, the slurry pH was 6.8. The powderobtained was an iron sulfide having a mackinawite structure, in whichthe molar proportions of the respective components were represented byFeCa_(0.008)Na_(0.14)S_(0.94). The powder which had undergone thedurability test was a mixture of sulfur and iron(II) oxide, which areproducts of decomposition of iron sulfide. No peak attributable to aniron sulfide was observed.

The results of Examples 1 to 5 and 12 to 15 show that the iron sulfideshaving a denatured mackinawite structure had better durability than theiron sulfides having a mackinawite structure. The results of ComparativeExample 4 show that when the slurry pH after ingredient addition waslower than 7, the presence of calcium during iron sulfide preparationdid not result in incorporation of calcium and in an improvement indurability.

Examples in which iron sulfides of the invention were used to treatheavy metals are shown below together with the results thereof.

EXAMPLE 16

An alkaline fly ash containing 2,400 ppm lead, 160 ppm chromium, and 2.1ppm mercury was used to examine the property of treating heavy metals.To 100 parts by weight of the alkaline fly ash were added 30 parts byweight of water and various amounts of each of the iron sulfidesprepared in Examples and Comparative Examples. The resulting mixture waskneaded to conduct heavy metal treatment. The fly ash thus treated wassubjected to the leaching test in accordance with Japanese EnvironmentAgency Notification No. 13 (1973). The results obtained are shown inTable 2.

TABLE 2 Amount of iron sulfide added Pb Cr Hg Kind of iron sulfide(parts by leaching leaching leaching Used weight) (ppm) (ppm) (ppb) Noiron sulfide 0 98 0.2 3.6 (blank) Example 1, before 3 14 0.02 0.8durability test Example 1, before 4 3.4 <0.01 <0.5 durability testExample 1, before 5 <0.05 <0.01 <0.5 durability test Example 1, after 5<0.05 <0.01 <0.5 durability test Example 5, before 5 <0.05 <0.01 <0.5durability test Example 5, after 5 <0.05 <0.01 <0.5 durability testExample 6, before 5 <0.05 <0.01 <0.5 durability test Example 6, after 5<0.05 <0.01 <0.5 durability test Example 7, before 5 <0.05 <0.01 <0.5durability test Example 7, after 5 <0.05 <0.01 <0.5 durability testExample 8, before 3 16 0.02 0.8 durability test Example 8, before 4 2.8<0.01 <0.5 durability test Example 8, before 5 <0.05 <0.01 <0.5durability test Example 8, after 5 <0.05 <0.01 <0.5 durability testExample 9, before 5 <0.05 <0.01 <0.5 durability test Exampe 9, after 5<0.05 <0.01 <0.5 durability test Example 10, before 5 <0.05 <0.01 <0.5durability test Example 10, after 5 <0.05 <0.01 <0.5 durability testExample 11, before 5 <0.05 <0.01 <0.5 durability test Example 11, after5 <0.05 <0.01 <0.5 durability test Comparative Example 1, 5 <0.05 <0.01<0.5 before durability test Comparative Example 1, 5 71 0.07 1.2 afterdurability test Comparative Example 3, 5 45 0.05 0.8 before durabilitytest Comparative Example 3, 10  31 0.03 <0.5 before durability testComparative Example 4, 5 <0.05 <0.01 <0.5 before durability testComparative Example 4, 5 36 0.06 1.1 after durability test

The results given in Table 2 show that the heavy metal treatingproperties of the iron sulfides obtained in the Examples which had notundergone the durability test were almost equal to those of themackinawite iron sulfides heretofore in use and were far higher thanthose of the pyrrhotite iron sulfide obtained in a Comparative Example.The results further show that the mackinawite iron sulfides heretoforein use showed deteriorated heavy metal treating properties after the oneday durability test at 70° C. and 70% RH, whereas the iron sulfidesaccording to the invention retained their heavy metal treatingproperties after the durability test.

EXAMPLE 17

A neutral fly ash containing 1,900 ppm lead, 1,100 ppm chromium and 100ppm cadmium was used to examine the property of treating heavy metals.To 100 parts by weight of the neutral fly ash were added 30 parts byweight of water and various amounts of each of the iron sulfidesprepared in the Examples and Comparative Example. The resulting mixturewas kneaded to conduct heavy metal treatment. The fly ash thus treatedwas subjected to the leaching test in accordance with JapaneseEnvironment Agency Notification No. 13 (1973). The results obtained areshown in Table 3.

TABLE 3 Amount of iron sulfide Pb Cr Cd Kind of iron sulfide added(parts leaching leaching leaching used by weight) (ppm) (ppm) (ppm) Noiron sulfide — 18 0.03 3.8 (blank) Example 8, before 3 3.7 0.02 0.32durability test Example 8, before 4 0.1 <0.01 0.02 durability testExample 8, before 5 <0.05 <0.05 <0.01 durability test Example 8, after 5<0.05 <0.05 <0.01 durability test Example 9, before 5 <0.05 <0.05 <0.01durability test Example 9, after 5 <0.05 <0.05 <0.01 durability testComparative Example 1, 5 <0.05 <0.05 <0.01 before durability testComparative Example 1, 5 12 0.02 2.1 after durability test

The results given in Table 3 show that the iron sulfides obtained inExamples 8 and 9 were effective also in the treatment of heavy metalscontained in a neutral fly ash. The results further show that these ironsulfides were almost equal in initial treating properties to thesynthetic iron sulfide of Comparative Example 1 heretofore in use andhad far higher durability than it.

EXAMPLE 18

A contaminated model soil (water content, 50 wt %) containing 7,700 ppmlead, 470 ppm cadmium, 1,800 ppm hexavalent chromium, 96 ppm arsenic and2,200 ppm selenium was used to examine the property of treating heavymetals. To 100 parts by weight of the soil was added each of the ironsulfides prepared in Examples and a Comparative Example. The resultantmixture was kneaded to conduct heavy metal treatment. The soil thustreated was subjected to the leaching test in accordance with JapaneseEnvironment Agency Notification No. 46 (1991). The results obtained areshown in Tables 4 and 5.

TABLE 4 Amount of iron sulfide Pb Cd Cr Kind of iron sulfide added(parts leaching leaching leaching Used by weight) (ppm) (ppm) (ppm) Noiron sulfide — <0.05 13 6.0 (blank) Example 1, before 10 <0.05 <0.010.02 durability test Example 1, after 10 <0.05 <0.01 0.02 durabilitytest Example 8, before 10 <0.05 <0.01 0.02 durability test Example 8,after 10 <0.05 <0.01 0.02 durability test Comparative Example 1, 10<0.05 <0.01 <0.01 before durability test Comparative Example 1, 10 <0.054.6 1.9 after durability test

TABLE 5 As Se Kind of iron sulfide leaching leaching used (ppm) (ppm) Noiron sultide 0.03 1.1 (blank) Example 1, before <0.01 0.06 durabilitytest Example 1, after <0.01 0.08 durability test Example 8, before <0.010.06 durability test Example 8, after <0.01 0.08 durability testComparative Example 1, <0.01 0.05 before durability test ComparativeExample 1, <0.01 0.86 after durability Test

The results given in Tables 4 and 5 show that the iron sulfides obtainedin Examples 1 and 8 were effective also in the treatment of heavy metalscontained in a soil. The results further show that these iron sulfideswere almost equal in initial treating properties to the mackinawite ironsulfide obtained in Comparative Example 1 and had far higher durabilitythan the mackinawite iron sulfide.

EXAMPLE 19

To each of six model wastewaters, i.e., a solution containing 10 ppmlead, solution containing 10 ppm cadmium, solution containing 1 ppmmercury, solution containing 10 ppm hexavalent chromium, solutioncontaining 10 ppm arsenic and solution containing 10 ppm selenium, wasadded 0.2 parts by weight of each of the iron sulfides prepared inExamples 1 and 8 to examine the heavy metal treating properties of theiron sulfides. After the iron sulfides were added to the modelwastewaters, each resulting mixture was stirred for 30 minutes and thenfiltered through a glass filter paper (GS-25, manufactured by AdvantecToyo Co.). The amounts of the heavy metals contained in the filtrateswere determined. The results obtained are shown in Table 6.

TABLE 6 Heavy metal Heavy metal amount after amount after treatment withtreatment with iron sulfide iron sulfide of Example 1 of Example 8 Kindof model wastewater (ppm) (ppm) Lead 10 ppm solution <0.05 <0.05 Cadmium10 ppm solution <0.01 <0.01 Mercury 1 ppm solution <0.005 <0.005Hexavalent chromium 10 ppm <0.01 <0.01 Solution Arsenic 10 ppm solution0.37 0.39 Selenium 10 ppm solution 0.17 0.15

The results given in Table 6 show that the iron sulfides according tothe invention were effective also in the treatment of heavy metalscontained in wastewaters.

The invention produces the following effects.

1) The iron sulfides of the invention have a mackinawite structurehaving a novel composition which has not been known. They have excellentheavy metal treating properties and excellent durability.

2) The processes of the invention can easily produce the excellent ironsulfides.

3) The heavy metal treating agent of the invention, which compriseseither of the excellent iron sulfides as an effective component, iseffective in treating heavy metals contained in ashes, soils,wastewaters, etc. to make them harmless.

1. An iron sulfide mixture comprising an iron sulfide selected from thegroup consisting of (a), (b), (c) and (d) below and at least onealkaline earth metal compound: (a) an iron sulfide with excellentdurability having a mackinawite structure which containsFeM_(x)N_(y)S_(z) wherein M represents an alkaline earth metal, Nrepresents an alkali metal, and x, y, and z, indicating the molarproportions of the respective elements, represent numbers satisfying0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component; (b) an ironsulfide with excellent durability having a mackinawite structure whichcontains FeM′_(x)N_(y)S_(z) wherein M′ represents Ca, Mg or combinationthereof, N represents an alkali metal, and x, y and z, indicating themolar proportions of the respective elements, represent numberssatisfying 0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component;(c) an iron sulfide with excellent durability having a mackinawitestructure which contains FeM_(x)N_(y)S_(z) wherein M represents analkaline earth metal, N represents an alkali metal, and x, y and z,indicating the molar proportions of the respective elements, representnumbers satisfying 0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essentialcomponent, wherein the mackinawite structure is a denatured mackinawitestructure which gives an XRD pattern wherein spacing between 001 planeshas increased in c axis direction to a value of from 5.03 A to 5.53 Aand the ratio of the intensity for the diffraction peak attributable toany other hkl planes to that for the diffraction peak attributable tothe 001 planes is 20/100 or lower; and (d) an iron sulfide withexcellent durability having a mackinawite structure which containsFeM′_(x)N_(y)S_(z) wherein M′ represents Ca, Mg or combination thereof,N represents an alkali metal, and x, y and z, indicating the molarproportions of the respective elements, represent numbers satisfying0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component; wherein themackinawite structure is a denatured mackinawite structure which givesan XRD pattern wherein spacing between 001 planes has increased in caxis direction to a value of from 5.03 A to 5.53 A and the ratio of theintensity for the diffraction peak attributable to any other hkl planesto that for the diffraction peak attributable to the 001 planes is20/100 or lower.
 2. The iron sulfide mixture as claimed in claim 1,wherein the alkaline earth metal compound is at least one compoundselected from the group consisting of hydroxides, carboxylates,phosphates and sulfites of alkaline earth metals.
 3. A heavy metaltreating agent comprising the mixture of claim
 2. 4. A method fortreating heavy metals which comprises adding the heavy metal treatingagent of claim 3 to a refuse incineration ash, fly ash or fused fly asheach containing at least one heavy metal, and kneading the resultingmixture.
 5. The method as claimed in claim 4, wherein the heavy metal isat least one element selected from the group consisting of Pb, Cd, Hg,Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 6. The method as claimed in claim 4,wherein after adding the heavy metal treating agent, water is added tothe mixture.
 7. The method as claimed in claim 6, wherein the heavymetal is at least one element selected from the group consisting of Pb,Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 8. A method for treatingheavy metals which comprises adding the heavy metal treating agent ofclaim 3 to a soil containing at least one heavy metal, and kneading theresulting mixture.
 9. The method as claimed in claim 8, wherein theheavy metal is at least one element selected from the group consistingof Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 10. The method asclaimed in claim 8, wherein after adding the heavy metal treating agent,water is added to the mixture.
 11. The method as claimed in claim 10,wherein the heavy metal is at least one element selected from the groupconsisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 12. Amethod for treating heavy metals which comprises adding the heavy metaltreating agent of claim 3 to a waste water containing at least one heavymetal, and stirring the resulting mixture.
 13. The method as claimed inclaim 12, wherein the heavy metal is at least one element elected fromthe group consisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.14. A heavy metal treating agent comprising the mixture of claim
 1. 15.A method for treating heavy metals which comprises adding the heavymetal treating agent of claim 14 to a refuse incineration ash, fly ashor fused fly ash each containing at least one heavy metal, and kneadingthe resulting mixture.
 16. The method as claimed in claim 15, whereinthe heavy metal is at least one element selected from the groupconsisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 17. Themethod as claimed in claim 15, wherein after adding the heavy metaltreating agent, water is added to the mixture.
 18. The method as claimedin claim 17, wherein the heavy metal is at least one element selectedfrom the group consisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb andMo.
 19. A method for treating heavy metals which comprises adding theheavy metal treating agent of claim 14 to a soil containing at least oneheavy metal, and kneading the resulting mixture.
 20. The method asclaimed in claim 19, wherein the heavy metal is at least one elementselected from the group consisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As,Se, Sb and Mo.
 21. The method as claimed in claim 19, wherein afteradding the heavy metal treating agent, water is added to the mixture.22. The method as claimed in claim 21, wherein the heavy metal is atleast one element selected from the group consisting of Pb, Cd, Hg, Zn,Cu, Ni, Cr, As, Se, Sb and Mo.
 23. A method for treating heavy metalswhich comprises adding the heavy metal treating agent of claim 14 to awaste water containing at least one heavy metal, and stirring theresulting mixture.
 24. The method as claimed in claim 23, wherein theheavy metal is at least one element selected from the group consistingof Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 25. A heavy metaltreating agent comprising an iron sulfide composition comprising an ironsulfide selected from the group consisting of (a), (b), (c) and (d)below as an effective component: (a) an iron sulfide with excellentdurability having mackinawite structure which contains FeM_(x)N_(y)S_(z)wherein M represents an alkaline earth metal, N represents an alkalimetal, and x, y and z, indicating the molar proportions of therespective elements, represent numbers satisfying 0.01<x≦0.5, y≦0.2 and0.7≦z≦1.4, as an essential component; (b) an iron sulfide with excellentdurability having a mackinawite structure which containsFeM′_(x)N_(y)S_(z) wherein M′ represents Ca, Mg or combination thereof,N represents an alkali metal, and x, y and z, indicating the molarproportions of the respective elements, represent numbers satisfying0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component; (c) an ironsulfide with excellent durability having a mackinawite structure whichcontains FeM_(x)N_(y)S_(z) wherein M represents an alkaline earth metal,N represents an alkali metal, and x, y and z, indicating the molarproportions of the respective elements, represent numbers satisfying0.01<x≦0.5, y≦0.2 and 0.7≦z≦1.4, as an essential component, wherein themackinawite structure is a denatured mackinawite structure which givesan XRD pattern wherein spacing between 001 planes has increased in caxis direction to a value of from 5.03 A to 5.53 A and the ratio of theintensity for the diffraction peak attributable to any other hkl planesto that for the diffraction peak attributable to the 001 planes is20/100 or lower; and (d) an iron sulfide with excellent durabilityhaving a mackinawite structure which contains FeM′_(x)N_(y)S_(z) whereinM′ represents Ca, Mg or combination thereof, N represents an alkalimetal, and x, y and z, indicating the molar proportions of therespective elements, represent numbers satisfying 0.01<x≦0.5, y≦0.2 and0.7≦z≦1.4 as an essential component; wherein the mackinawite structureis a denatured mackinawite structure which gives an XRD pattern whereinspacing between 001 planes has increased in c axis direction to a valueof from 5.03 A to 5.53 A and the ratio of the intensity for thediffraction peak attributable to any other hkl planes to that for thediffraction peak attributable to the 001 planes is 20/100 or lower. 26.A method for treating heavy metals which comprises adding the heavymetal treating agent of claim 25 to a refuse incineration ash, fly ashor fused fly ash each containing at least one heavy metal, and kneadingthe resulting mixture.
 27. The method as claimed in claim 26, whereinthe heavy metal is at least one element selected from the groupconsisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.
 28. Themethod as claimed in claim 26, wherein after adding the heavy metaltreating agent, water is added to the mixture.
 29. The method as claimedin claim 28, wherein the heavy metal is at least one element selectedfrom the group consisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb andMo.
 30. A method for treating heavy metals which comprises adding theheavy metal treating agent of claim 25 to a soil containing at least oneheavy metal, and kneading the resulting mixture.
 31. The method asclaimed in claim 30, wherein the heavy metal is at least one elementselected from the group consisting of Pb, Cd, Hg, Zn, Cu, Ni, Cr, As,Se, Sb and Mo.
 32. The method as claimed in claim 30, wherein afteradding the heavy metal treating agent, water is added to the mixture.33. The method as claimed in claim 32, wherein the heavy metal is atleast one element selected from the group consisting of Pb, Cd, Hg, Zn,Cu, Ni, Cr, As, Se, Sb and Mo.
 34. A method for treating heavy metalswhich comprises adding the heavy metal treating agent of claim 25 to awaste water containing at least one heavy metal, and stirring theresulting mixture.
 35. The method as claimed in claim 34, wherein theheavy metal is at least one element selected from the group consistingof Pb, Cd, Hg, Zn, Cu, Ni, Cr, As, Se, Sb and Mo.